1. Field of Invention
The invention relates to equalizing structures arranged to compensate for variations in the phase and attenuation characteristics of transmission lines and other pieces of apparatus; and more specifically, to the type of attenuation equalizer sometimes referred to as a Bode equalizer and as disclosed in U.S. Pat. No. 2,096,027.
2. Description of Prior Art
Ordinarily equalizer circuits have definite characteristics determined by the particular apparatus with which they are associated. Frequently, however, the equalizing requirements cannot be determined in advance either because the characteristics of the associated apparatus are not known with sufficient precision or because they vary with time. Examples are found in the equalization of transmission lines where the exact length is not known, or where the characteristics may be affected by changes in temperature and humidity. In such instances, an adjustable equalizer is required. The Bode equalizer is of this type.
The Bode equalizer has, inter alia, two desirable features:
A. The adjustment is accomplished by varying resistors only; and PA1 B. The correction remains proportional, i.e., a design that is proper for equalizing 2,000 feet of cable will automatically be correct, when the resistors are adjusted, for equalizing the proportional attenuation shape resulting from 1,000 feet of cable, or even a negative length (i.e., cable simulation). PA1 A. All Bode networks contain one or more inductors; and PA1 B. when set to produce no equalization, the more useful Bode networks have a constant insertion loss proportional to the maximum correction envisioned.
The Bode networks have, among others, two disadvantages which are:
The presence of this constant component of the insertion loss may significantly reduce the signal-to-noise ratio. The use of preamplifiers to boost the signal level and so to overcome the inherent attenuation of the Bode equalizer may thus sometimes be necessary.
More recently, the analysis of active variable equalizers has been discussed by Franc Brglez in a paper entitled, "Inductorless Variable Equalizers Using All-Pass Network Structures," Proceedings 1974, IEEE International Symposium on Circuits and Systems, Apr. 22 - 25, 1974, San Francisco, California, at pages 448 - 452. A different variable equalizer structure is disclosed by the same author in a paper entitled, "Minimally Active RC Variable Equalizers," IEEE Transactions on Circuits and Systems, vol. CAS-22, No. 8, August 1975, at pages 688 - 691.